The present invention relates to a control system of a continuously variable belt-drive automatic transmission for a motor vehicle, and particularly to a hydraulic control system for the continuously variable transmission.
U.S. Pat. No. 4,400,164 discloses a control system for a continuously variable belt-drive transmission. The transmission comprises an endless belt running over a drive pulley and a driven pulley, each pulley is so designed that the running diameter of the driving belt on the pulleys varies by a hydraulic control system.
In the hydraulic control system, a line pressure of oil is continuously applied to a servo chamber for the driven pulley and further the line pressure is applied to another servo chamber for the drive pulley by a transmission ratio control valve after starting the operation so as to control to upshift the transmission. In the conventional system, before starting the upshift operation, the servo chamber of the drive pulley is communicated with a drain port of the transmission ratio control valve to drain the servo chamber, so as to provide a maximum low speed stage at a maximum transmission ratio.
However, if the servo chamber of the drive pulley is kept draining before the starting of the transmission ratio changing operation, all or a part of the oil in the servo chamber is discharged, resulting in induction of air into the servo chamber. Accordingly, when the line pressure is applied to the servo chamber of the drive pulley through the transmission ratio control valve, the air in the servo chamber is compressed first, and then the pressure of oil elevates to start the upshifting of the transmission, causing in delays in elevation of the oil pressure and in response of transmission operation.
Japanese Patent Laid Open No. 58-94663 discloses a control system for a continuously variable transmission intended to eliminate above described disadvantages. In the control system, auxiliary circuit is connected to a passage for a servo chamber of a drive pulley so as to supply low pressure oil to the servo chamber before the starting of upshifting of the transmission. When a spool of a transmission ratio control valve is shifted, oil at a line pressure is supplied to the servo chamber to start upshifting. Since the line pressure is higher than the low pressure of oil in the auxiliary circuit, the line pressure oil passes through the auxiliary circuit in the counter direction. In order to prevent the counter flow of the oil, a check valve must be provided in the auxiliary circuit, which renders the construction of the system complicated.